The endothelium (lining of blood vessels) is capable of releasing endothelium-derived relaxant factor, a substance that is capable of causing relaxation of blood vessels and also inhibits blood clotting by antagonizing platelets from aggregating. These findings have been demonstrated in animal studies, but in our study we have shown that stimulating the release of endothelium-derived relaxant factor can cause not only relaxation of blood vessels, but can lead to inhibition of platelet aggregation. However, in patients with atherosclerosis (hardening of blood vessel), the capacity of these blood vessels to cause inhibition of platelets is much reduced.